Colombia's right-wing paramilitaries say they'll withdraw from peace talks and return to war unless the government changes proposed amnesty legislation.

The United Self Defence Forces of Colombia, the AUC, issued its ultimatum, saying the government was missing a historic chance for peace.

Paramilitary leaders say the legislative framework making its way through Congress is too severe.

The US and the UN say it is too lenient and leaves loopholes.

The AUC says the new law would still leave them open to extradition to the United States on drugs charges, where many have warrants pending on drugs trafficking charges.

Strategy

The paramilitary spokesman, Ernesto Baez, made the statement with the other AUC commanders alongside him, all heavily armed.

He said that the days of the peace process were numbered and that the paramilitary troops would return to the mountains and start fighting again.

The AUC has rejected the amnesty legislation that's currently making its way through congress.

The justice and peace bill stipulates that paramilitary commanders guilty of war crimes could spend up to eight years in prison.

There is no provision to protect AUC leaders from extradition, a relevant issue since some 10 paramilitary commanders have extradition warrants to the US pending on drugs trafficking charges.

Politicians commenting on the paramilitary declaration said that the AUC is simply trying to blackmail congress.

The United Nations and human rights groups have already criticised the proposed legislation as too lenient, meaning that the government is unlikely to soften its position.

Until now, the 16,000-strong AUC has restricted its war to the Marxist guerillas.

However, should the peace process fail, there are fears it could turn its guns on a beleaguered government.

Efforts to disarm the brutal AUC, responsible for some of the worst atrocities of the 41-year civil conflict, has been the centrepiece of President Alvaro Uribe's strategy to bring an end to the violence.